Process for electrolytic hydrodimerization of alpha beta-unsaturated compounds

ABSTRACT

THE PROCESS OF ELECTROLYTIC HYDRODIMERIZATION OF ALPHA, BETA-OLEFINICALLY UNSATURATED COMPOUNDS IN A CONDUCTIVE AQUEOUS MEDIUM CONSISTING ESSENTIALLY OF A HYDRODIMERIZABLE ALPH,BETA-OLEFINICALLY UNSATURATED COMPOUND DISSOLVED IN AN AQUEOUS, CONDUCTIVE SALT SOLUTION WHEREIN AN ALTERNATING CURRENT IS PASSED BETWEEN ELECTRODES AT A FREQUENCY THAT WILL MAINTAIN A PRESELECTED PH RANGE AND MINIMIZE BY-PRODUCT FORMATION.

United States Patent 3,647,651 PROCESS FOR ELECTROLYTIC HYDRODI- MERIZATION 0F ALPHA,BETA-UNSATU- RATED COMPOUNDS James B. Ganci, Wilmington, and Sebastian V. R. Mastrangelo, Hockessin, Del., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del. No Drawing. Filed Aug. 31, 1970, Ser. No. 68,497 Int. Cl. C07b 29/06; C07c 121/02, 121/26 US. Cl. 20473 A 4 Claims ABSTRACT OF THE DISCLOSURE The process of electrolytic hydrodimerization of alpha, beta-olefinically unsaturated compounds in a conductive aqueous medium consisting essentially of a hydrodimerizable alpha,beta-olefinically unsaturated compound dissolved in an aqueous, conductive salt solution wherein an alternating current is passed between electrodes at a frequency that will maintain a preselected pH range and minimize by-product formation.

INTRODUCTION This invention relates to the hydrodimerization of alpha,beta-unsaturated compounds to obtain saturated dimers thereof. More specifically, this invention relates to an improved electrolytic process for the hydrodimerization of alpha,beta-unsaturated compounds through the use of alternating current.

The direct current electrolytic hydrodimerization of alpha,beta-olefinically unsaturated compounds in a conductive aqueous medium is a well-known process for preparing the beta,beta-reductively coupled dimers. The process is commerciallypractical, for example, in the manufacture of the hydrodimer, adiponitrile, from the alpha,beta-unsaturated acrylonitrile. Such electrolytic processes are exemplified in U.S. Pats. 3,193,480, 3,193,- 481, 3,193,482 and 3,193,483.

The prior art indicates that it is important to control hydrogen ion concentration in the electrolyte utilized in the electrolytic cells employed. Such control is necessary to minimize undesirable side reactions of the a1pha,betaunsaturated monomers. These side reactions are promoted by excessively acidic or excessively basic conditions in the vicinity of the electrodes of the electrolytic cells during electrolysis due to hydroxyl ion concentration increase in the vicinity of the cathode and hydrogen ion concentration increase in the vicinity of the anode.

The prior art indicates that build ups in acidic and basic conditions in undivided electrolytic cells can be overcome by thorough agitation. Another way to overcome this problem is to exclude the alpha,=beta-unsaturated compound from the anolyte by employing a membrane divided cell. However, at the cathode of such a cell hydroxyl ion concentration still increases and must be controlled during electrolysis to avoid undesirable side reactions. The catholyte pH can be kept from becoming too high by bleeding acid through the membrane from anolyte to catholyte or by the addition of acid to the catholyte to maintain a preselected pH range. In the bulk of the catholyte, for example, a neutral or slightly basic catholyte will minimize side reactions. However, in such cases there is still a net increased concentration of hydroxyl ions at the cathode which vigorous agitation cannot completely overcome.

The divided cell prior art processes are, however, cumbersome. Two difierent electrolytes are required. A membrane is required which can add significantly to the IR drop within the cell and therefore cause unnecessary heating of cell contents. Further, rectification or partial rectification of ordinary alternating current is required to provide the direct current or direct current biased with alternating current disclosed in the prior art. The installation and maintenance of rectification equipment is costly and adds to equipment complexity.

An object of this invention is to provide an improved process for the hydrodimerization of alpha,beta-unsaturated compounds. Another object of this invention is to provide an electrolytic process for hydrodimerization of alpha,beta-unsaturated compounds by using alternating current. Still another object of this invention is to provide an improved process for hydrodimerization of alpha,beta-unsaturated compounds whereby the need for a divided electrolytic cell, the use of two different electrolytes, extraneous means for adjusting pH, and the need for rectification of alternating current are eliminated.

SUMMARY OF THE INVENTION Now in accordance with the invention an improved process for the electrolytic hydrodimerization of alpha, beta-olefinically unsaturated compounds has been discovered wherein a hydrodimerizing alternating current is passed between electrodes separated by an electrolyte consisting essentially of a hydrodimerizable alpha,betaolefinically unsaturated compound dissolved in an aqueous, conductive salt solution with the frequency of the alternating current set to maintain, at each electrode, a preselected pH range.

DESCRIPTION OF THE INVENTION The novel process of this invention comprises passing a hydrodimerizing alternating electric current between electrodes separated by and in electrical contact with a liquid electrolyte consisting essentially of a hydrodimerizable alpha,beta-olefinically unsaturated compound dissolved in an aqueous, conductive salt solution. As the alternating current flows through the electrolyte, each electrode becomes, in turn, cathodic and then anodic. When, during a half-cycle, an electrode is cathodic, multiples of two molecules of the alpha,beta-unsaturated compounds are coupled and reduced at or near the electrode. The product is the hydrodimer. Thus, the electrolytic hydrodimerization of acrylonitrile is accomplished by the process of this invention by dissolving acrylonitrile in an aqueous tetraalkyl ammonium toluene sulfonate solution and passing 60 cycle alternating current between the mercury electrodes in contact with the acrylonitrile containing solution and obtaining adiponitrile of very high purity without the undesirable side reactions caused by increased hydroxyl ion concentration at the cathode and increasing hydrogen ion concentration at the anode.

The process of this invention can be illustrated by the following equation of cathodic reaction wherein two molecules of the alpha,beta-unsaturated monomer, acrylonitrile, provide a molecule of the hydrodimer, adiponitrile.

In the anode reaction which occurs during the other half-cycle of the alternating current, water is oxidized to provide hydrogen ions. This reaction may be illustrated by the following equations where M is anode material oxidizable under the electrolysis conditions. These hydrogen ions are produced in quantity stoichiometrically equivalent to the hydroxyl ions produced during the cathodic portion of the cycle. Thus by utilizing alternating current, hydrogen and hydroxyl ions are alternately produced at the same electrode, and the hydrogen and hydroxylions are, therefore, available for reaction with one another in the immediate vicinity of the electrode. Thus, there is no need to transport hydrogen ion from one electrode side of the cell to the other as in the prior art direct current processes.

In the process of the invention, the electrode boundary layer concentration of hydrogen or hydroxyl ions is disrupted within each cycle of the alternating current, and where the hydrodimerizing alternating current frequency is properly chosen, any desired preselected pH range can be maintained at the electrodes, and, of course, in the bulk of the electrolyte. Thus, as in the process of this invention, where a range in pH of 3 to 12 is operable and a substantially neutral to mildly alkaline pH range of 6 to 9.5 is preferred, the present process permits maintaining such pH ranges at the electrodes where pH control is most critical.

In the process of this invention substantially alternating current is used to control the pH of the electrolyte at the electrodes. The importance of this achievement is that the pH control is located at the electrode-liquid surface which is where the reaction is occurring and hence of primary importance. The pH control is accomplished by the control of the frequency. The frequency of the alternating current will maintain a preselected range of pH wherein by-product formation during hydrodimerization of the alpha,beta-unsaturated compound is minimized. It is therefore clear that the process of this invention is directed to a process whereby pH control at the electrodes can be conveniently controlled by the frequency of the alternating current to minimize by-produet formation during hydrodimerization of the alpha,beta-unsaturated compounds of this invention.

The alpha,beta-unsaturated compounds, also referred to herein as monomers, which are hydrodimerizable by the process of this invention include, generally, any monoolefin having at least one activating substituent attached to one of the olefinically unsaturated carbon atoms, and which monomer is hydrodimerizable under the direct current conditions described in the art. They include compounds disclosed in US. Pats. 3,193,480, 3,193,481, 3,193,482 and 3,193,483. Activating substituents are, for example, those having carbonyl or nitrile function,

H CEN Representative examples of the monomers which can be hydrodimerized by the process of this invention are acrylonitrile, .methacrylonitrile, crotonitrile, Z-methylenebutyronitrile, Z-pentenenitrile, fumaronitrile, methyl acrylate,

isopropyl methacrylate, butyl crotonate, dimethyl maleate,

diethyl fumarate, methyl 2-isopropylcrotonate, ethyl 2- penteneoate, methylvinyl ketone, acrylamide, N-N-propylacrylamide, N,N-dimethylacrylamide, N,Ndiethylcrotonamide, N,N,N',N-tetramethylfumaramide and the like.

Because of the considerable commercial importance of adipontrile, acrylonitrile is an important starting monomer for the present process. In the process of this invention when acrylonitrile is the starting monomer, a pH range of from 6 to 9.5 is preferred.

The term hydrodimerizing alternating current as employed herein broadly encompasses any electric current consisting essentially of alternating current wherein the direction of current flow is periodically reversed, andwhich provides sufficient current to effect the desired electrochemical hydrodimerization. It will be appreciated that the more nearly the periodicity of the current flow approches regularity, i.e., approaches a point Where i t, (amperes multipled by time) is equal but opposite in sign for each direction of current fiow, the less will be the tendency to produce an excessive variation in acidity or alkalinity at the electrodes of electrolytic cells utilized in this process. Thus, some direct current bias may be placed on the alternating current utilizedin this invention.

The only limitation is that such bias not be sufiicient to effect, in a practical period of time, a change in pH at the electrodes, which change is outside preselected pH limits.

Alternating current frequencies useful in this invention vary widely, the only limits being practical ones relating, for example, to too low a frequency to avoid excessive pH changes at electrolytic cell electrodes and frequencies above which the rate of hydrodimerization becomes impractically slow. It is virtually impossible to assign numerical values to such frequency limits because they depend on many more or less interrelated variables such as current density, rate of hydrodimerization and the nature of the alpha,beta-unsaturated monomers, the supporting electrolyte and the electrode materials. However, it is very easy to determine both operable and practical frequency limits quantitatively. For example, with any given cell, conductive salt solution, monomer, monomer concentration and current density, one can readily arrive at operable and practical frequency ranges by operating the cell for a selected time at different frequencies and, for each operating period, determining hydrodimer production and hydrodimer purity. Hydrodirner purity is merely the percentage of hydrodimer to the hydrodimer with by-products formed. Frequencies that are not high enough to maintain the pH Within the preselected range at the electrodes will be indicated by increased quantities of the by-products produced, thereby lowering the hydrodimer purity. Too high frequencies will be indicated by poor productivity of hydrodimer. If the frequency is ex tremely high, little hydrodimer will be produced. Thus, the alternating current frequently can be easily selected to produce an optimum pH range for any given monomer providing a maximum productivity.

Such well-known analytical procedures as vapor phase chromatography can be readily employed to determine the quantitative ratios of hydrodimer-to-impurities. Productivity is determined quite simply by recovering and weighing the hydrodimer produced in the cell during the operating period at a selected frequency. The hydrodimerized product may be separated from the reaction mixture by separation procedures known to those skilled in the art, for example, by extraction, fractionation, etc. Generally the reaction mixture is extracted with organic solvent and the organic phase is separated by decanting. After removing any residual inorganic matter by washing with water, the organic material is distilled to remove solvent leaving as residue the hydrodimerization product and any unconverted alpha,beta-unsatura'ted compounds, together with by-produucts if any. These may be separated from each other, for example, by fractional distillation, etc. In experimental runs, results of the electrolysis can be conveniently arrived at, when the products are liquid, simply by analytical determination of the hydrodimerization product and of the unconsumed monomer by vapor phase chromatography.

Still other art-known means can be adapted to determine operable and preferred frequencies. For example, the pH may be monitored in the close proximity of an electrode by means of a pH meter utilizing a micro pH probe. Alternatively, colorimetric procedures can be employed to detect color changes in an electrolyte soluble, e.g., by utilizing non-interfering acid-base indicator at electrode surfaces. As will be appreciated by those skilled in the art, such procedures can also be applied, for example, in a continuous process to monitor any tendency of electrolyte at pH electrodes to drift outside preselected limits, for example, under the influences of excessive direct current bias.

While alternating current frequencies of 400 c.p.s. and greater may be operable in the process of this invention, practical considerations of cost, commercial and ubiquitous availability indicate that the utilizationof 50 to 60 c.p.s. alternating current is preferred. Such current normally has a sinusoidal wave form which is, when illustrated graphically, symmetrical with respect to a zero or null amperage-time axis and to a zero or null voltagetime axis. Conventionally such amperage and voltage is expressed as the root-mean-square value (RMS). This convention will be employed hereinafter.

Voltages (RMS) required to provide hydrodimerizing alternating current vary widely depending on the electrode area in the electrolytic cells, the conductivity of the electrolyte and the electrode spacing. What is important is simply to provide sufiicient voltages to produce a desired current density. The current density is the amperes (RMS) per unit area of electrode surface. In the process of this invention, as little as about 1 ampere (RMS) or even less, per square decimeter of electrode surface may be operable for hydrodimerization. Since current density is related to productivity, generally the higher the current density the greater the production of hydrodimer. Depending on electrode spacing and electrolyte conductivity, upper current densities of 70, 100 amperes (RMS) per square decimeter or even higher can be very effectively employed especially if a means is provided to help dissipate any excessive heat produced at the higher current densities so that the electrolyte components are not, for example, evaporated or subjected to degradation temperatures. With the highly useful mercury electrodes current densities of about 20 to 70 amperes (RMS) per square decimeter are preferred due to the fact that such current densities do not require excessive cooling means.

This invention is not directed to electrolytic cell design. An electrolytic cell useful in the process of this invention would be obvious to those skilled in the art. By way of illustration of electrolytic cell design, one can employ simple cells with two electrodes or cells with multiple electrodes connected in series or parallel. Another type of cell arrangement can be a multi-phase cell where, for example, three electrodes are operably connected to each of the secondary windings of a three-phase transformer. In such arrangement, during cell operation, voltages and amperages vary continuously, symmetrically and regularly from positive to negative at a phase separation of 120. Regardless of which cell design is utilized, it is desirable to provide means of agitating and cooling the conductive liquid. Additionally for continuous operation, a means for continuously introducing the alpha, beta-unsaturated compound and continuously removing the hydrodimerized product should be provided.

Although almost any art-known, water-soluble, conductive salt may be operable, it is preferred that the electrolyte solution employed in this invention contain a conductive salt which is hydrotropic, i.e., which promotes the solubility of the alpha,beta-unsaturated monomer in the aqueous electrolyte. Among such hydrotropic salts are the amine salts of alkylsulfuric acids, the amine salts of aryl sulfonic acids, the quaternary alkyl ammonium salts of aryl sulfonic acids and the quaternary alkylammonium salts of alkyl sulfuric acids. The salts applicable to the process of this invention also include the salts disclosed in US. Pat. 3,193,480 beginning at column 4, line 24 to column 6, line 9. Representative examples of amine salts of alkyl sulfuric acids include mono-, diand trialkyl amine salts of such sulfuric acids, e.g., the ethyl amine, dimethyl amine, tributy amine, morpholine, piperidine and benzyl amine salts. Representative examples of the amine salts of aryl sulfonic acids include the amine salts of toluene sulfonic acids and the amine salts of xylene sulfonic acids. Representati-ve examples of the amine salts of alkyl sulfuric acids include triethyl amine salt of dodecyl sulfuric acid.

Representative examples of the quaternary alkyl ammonium salts of aryl sulfonic acids include tetraalkyl ammonium salts of benzene sulfonic acids, toluene sulfonic acids, xylene sulfonic acids and naphthalene sulfonic acids.

Representative examples of the quaternary alkyl ammonium salts of alkyl sulfuric acids include tetraethyl ammonium dodecyl sulfate.

The preferred hydrotropic salts are the tetraalkyl ammonium salts of aryl sulfonic acids. Representative examples of such preferred salts use tetramethyl-, tetraethyl-, tetrabutyl-, methyl triethyl and triethylbutyl ammonium salts of benzene sulfonic acids, ortho toluene sulfonic acids, meta toluene sulfonic acids, p-toluene sulfonic acids, alpha-naphthalene sulfonic acids, beta-naphthalene sulfonic acids and xylene sulfonic acids.

The preferred tetraalkyl ammonium salts of such sulfonic acids are particularly suitable for several reasons. The are relatively highly soluble in water and are highly hydrotropic. They are generally resistant to discharge in the electrolytic process of the invention, i.e., their discharge potential is generally more negative than that of the alpha,beta-unsaturated compounds. Further, they are inherently neutral and therefore easily used to prepare substantially neutral, e.g., pH 6 to 9.5 aqueous solutions. Particularly preferred because of availability, water solubility, inertness, high conductivity and high hydrotropicity are tetraalkyl ammonium salts of 0-, mand p-toluene sulfonic acids, or tetraalkyl ammonium salts of mixed toluene sulfonic acids, wherein each alkyl has from 1 to 4 carbon atoms. A very useful member of this group of salts is tetraethylammonium toluene sulfonate.

The electrolyte prepared from such salts should contain at least about 10% by weight of the salt and at least about 5% by weight of the alpha,beta-unsaturated monomer. Higher hydrotropic salt concentrations promote the solubility of the monomers. It is therefore preferred that the salt concentration in water approach saturation, e.g., 50 to 70% by weight of the tetraethylammonium toluene sulfonate dissoved in water. It is also preferred that said solution, in turn, be substantially saturated with respect to the alpha,beta-unsaturated compound. Thus high electrolyte conductivity and maximum monomer availability for hydrodimerization is assured. Excess monomer can be maintained in contact with the electrolyte to assure saturation.

Electrodes, suitable for the electrolytic cells used in the process of this invention, can be fabricated from a wide variety of art-known conductive materials. Representative examples of electrode materials are, for example, carbon, copper, stainless steel, mercury, zinc, tin, cadmium, bismuth, lead, etc. In general, those of higher hydrogen overvoltage are preferred, although those of lower hydrogen overvoltage can also be used, even if they cause the generation of hydrogen under electrolysis conditions, as is the case with stainless steel and other electrodes of lower hydrogen overvoltage. It will be realized that hydrogen overvoltage can vary with the type of surface and prior history of the metal, as well as other factors. Therefore, the term hydrogen overvoltage as used herein with respect to copper as a gauge has reference to the overvoltage under the conditions of use in electrolysis. Metals having relatively high hydrogen overvoltages under the electrolytic condtions are preferred to avoid the cathodic production of hydrogen at the expense of hydrodimerization. Particularly suitable electrode metals are mercury, lead and tin. These have high hydrogen overvoltages are available commercially and are famililar electrode materials, the utilization of which is well known in the art. Of the three particularly suitable electrode metals, mercury is the most advantageous. As a high density liquid, mercury is easily kept free of the less dense, solid mercurous oxide formed during the alternating current process. For example, mercurous oxide can be removed for eventual recovery of mercury by simple agitation of the cell contents and filtration of the electrolyte; or for example, in a continuous system, by filtration of the mercury and recycling the filtered mercury to the electrode system of the cell. In addition, recovered mercurous oxide is relatively easily reconverted to metallic mercury by well-known means, e.g., by simple retorting.

One of the advantages of the present process is that electrodes can be and preferably are identical in composition, in shape and in disposition in an electrolytic cell. The cell construction and electrode manipulation useful in the process of this invention is therefore greatly simplified over that of direct current cells.

The following examples serve to further illustrate the novel process of this invention but are not in any way intended to limit the inventor.

Example 1 To a glass electrolytic cell provided with two mercury electrodes in the form of concentric annuli having an exposed area of 0.12 square decimeter each and spaced about 2 millimeters apart were added 109 grams of an aqueous neutral (pH-7) solution cantaining 60% by weight of tetraethylammonium toluene sulfonate and 42 grams of freshly distilled acrylonitrile. The cell contents were electrolyzed for 1 hour at 25 to 37 C. using a current density of 42 amperes (RMS) per square decimeter of electrode surface and a 60 cycles per second alternating current with voltages of 58 to 66 volts (RMS) to maintain the current density. Magnetic stirring was provided during the electrolysis to maintain Hg O in suspension in the electrolyte.

The cell contents were extracted with chloroform from which was isolated 2.9 grams of very pure adiponitrile. The adiponitrile was identified as such by infrared spectroscopy-and by comparison of its vapor phase chromatogram with that of an authenticated pure sample of adiponitrile. By-product impurities were substantially absent.

That pH remained practically neutral at the electrodes during the electrolysis as indicated by the substantial absence in the adiponitrile of polyacrylonitrile and bis-cyanoethyl ether, which formation is promoted respectively by excessively acidic and excessively basic conditions. Surprisingly, despite the expose of acrylonitrile to possible anodic oxidation, there was no evidence of acrylonitrile oxidation products in the recovered adiponitrile.

Examples 2 and 3 Two other electrolyses conducted as described in Example 1 demonstrate the use of lower and high current densities.

Current density, Mainteam nance Adiponitrile (RMS voltage, Time, Temp., recovered, Example dm. v. (RMS) hr. O. g.

The adiponitrile recovered was of about 95% purity as determined by Gas Chromatogram.

Similar results can be obtained in the hydrodimerization of methacrylonitrile, crotonitrile, acrylamide, butyl crotonate and like materials.

The following example demonstrates both the use of less concentrated salt solutions in the electrolyte and the use of lead electrodes.

Example 4 8. of the electrolyte solution and evaporation of the chloroform yielded pure adiponitrile. A white precipitate collected on the electrodes during the electrolysis. Similar results can be obtained with tin electrodes.

A mechanical means of keeping such electrodes free of oxide can be used and hydrodimer yield increased thereby The saturated dimers of alpha,beta-olefins which result from the process of this invention are useful in the manufacture of high molecular weight condensation polymers useful as fibers, filaments and molded products and are efficient plasticizers for synthetic resins and plastics.

The foregoing detailed description has been given for clarity of understanding only and no unnecessary limitations are to be understood therefrom. The invention is not limited to exact details shown and described for obvious modifications will occur to one skilled in the art.

What is claimed is:

1. In the electrolytic process for hydrodimerization an alpha,beta-olefinically unsaturated compound in an electrolyte consisting essentially of said unsaturated compound dissolved in an aqueous conductive salt solution, by passing a hydrodimerizing current between electrodes which are in contact with said electrolyte and recovering the saturated dimers thereof which process is characterized by increasing hydroxyl ion concentration in the vicinity of the cathode and increasing hydrogen ion in the vicinity of the anode and in which process the alpha, beta-unsaturated compound is prone to undergo side reactions promoted by either excessive alkalinity or excessive acidity, the improvement which comprises passing a hydrodimerizing current consisting essentially of alternating current between said electrodes wherein the frequency of the alternating current is such that the pH of the electrolyte in the vicinity of the electrodes is maintained at a preselected range which minimizes by-product formation during the hydrodimerization of the alpha, beta-olefinically unsaturated compound.

2. The process of claim 1 wherein the conductive salt is hydrotropic and has a discharge potential more negative than that of the alpha,beta-unsaturated olefinic compound, the electrodes have a high hydrogen overvoltage, the electrolyte is substanially neutral to mildly alkaline and the alternating current having a frequency of less than about 400 cycles per second.

3. The process of claim 2 wherein the electrolyte is at least 5% by weight of acrylonitrile dissolved in a medium consisting essentially of at least 10% by Weight in water of a tetraalkyl ammonium toluene sulfonate where each alkyl has 1 to 4 carbon atoms, the electrodes are selected from one of the metals mercury, lead and tin and the hydrodimerized product is adiponitrile.

4. The process of claim 3 where the electrolyte is a saturated solution of acrylonitrile in a medium consisting essentially of about 50% to by weight in water of tetraethylammonium toluene sulfonate, the electrodes are mercury, the alternating current has a frequency of 50 to 60 cycles per second, and there is utilized a current density of 20 to 70 amperes (RMS) per square decimeter of electrode surface.

References Cited UNITED STATES PATENTS 3,481,846 12/1969 Davis 204-73 A 3,505,186 4/l97'0 Sarazin et al 204-73 R 3,556,961 l/1971 Bizot et a1 20473 A F. C. EDMUNDSON, Primary Examiner U.S. Cl. X.R. 

